This is really a good idea, based on the number of tailgaters that the car has attracted, in my experience! Trouble is, I'm trying to figure how to fit the following (I guess a really small font is needed) -
"If you can read this, dear chap, then it is obvious that one of two things has transpired: Either, A) you are a genuine car guy trying to have a closer look at a cool car - I respect that, but might I suggest we pull over in the name of safety and I will gladly discuss the machine in detail with you, perhaps even let your kid sit in it, provided that his trousers are not dirty. Or, B) you are driving a lesser sports car and in your haste to 'save face', as it were, you have found yourself in quite a predicament. Namely, you have tried to keep up and then pulled a Felipe Massa into Turn One (layman please understand the term 'outbraked yourself') and thusly you are scant moments away from finding out that an aluminum frame will bend horrendously when it meets a carbon tub. Luckily for both of us, being in the non-lesser sports car, I have at my disposal a twin turbocharged motor capable of putting real estate between us, shall we say, "pronto" in your car's Mother tongue."
If that doesn't fit, then maybe just "Back Off, Fizzy Root!".